Presently, studies are conducted for circuit schemes for performing digital signal processing of high-frequency signals accompanying progress in digital signal processing technology. As an example, there is a technique of performing discrete-signal processing on a received signal using switched capacitors. Moreover, as another example, a technique of signal processing featuring a filtering function of sampling and band-limiting a received signal at the same time, is proposed.
A conventional filter apparatus has a voltage-to-current conversion section converting sampling signals to current and a set of capacitors formed with a plurality of switched capacitors. This filter apparatus charges sampling signals in the capacitors in a fixed order and collectively extracts charge from a plurality of capacitors, thereby providing working effects including a mixing effect by down sampling and a filtering effect by gathering and averaging a plurality of capacitors (for example, see patent document 1). FIG. 1 shows the conventional filter apparatus disclosed in patent document 1.
The filter apparatus in FIG. 1 charges signal 21, which is sampled received signals 20 converted to current, into capacitors C, in a predetermined order, through switches controlled by control signals 31, 32 and 33, from DCU 34 that controls timings. Then, the filter apparatus collectively picks up a plurality of charges and converts them to voltage, to acquire output signal 26.
The filter apparatus is able to change the weights of filter coefficients by changing the ratio of capacitance between a plurality of capacitors from which charge is collectively picked up, thereby providing a filtering effect of different characteristics from a simple decimation filter.    Patent Document 1: United States Patent Application Publication No. 2003/0050027 Specification (FIG. 5)